P-Aryl ether

A technique based on ozonation, in contrast, provides information on the stmcture of the lignin side chain by degrading the aromatic rings (33). Thus the side chain of the dominant stmcture ia all native lignins, the arylglycerol—P-aryl ether moiety, can be obtained ia the form of erythronic and threonic acids. Ozonation proves to be an elegant method for determination of the stereospecificity ia lignin. 

In the presence of sulfide or sulfhydryl anions, the quinonemethide is attacked and a benzyl thiol formed. The P-aryl ether linkage to the next phenylpropane unit is broken down as a result of neighboring-group attack by the sulfur, eliminating the aryloxy group which becomes reactive phenolate ion (eq. 2). If sulfide is not present, a principal reaction is the formation of the stable aryl enol ether, ArCH=CHOAr. A smaller amount of this product also forms in the presence of sulfhydryl anion. 

Hammel KE, MD Mozuch, KA Jensen, PJ Kersten (1994) HjOj recycling during oxidation of the arylglycerol P-aryl ether lignin structure by lignin peroxidase and glyoxal oxidase. Biochemistry 33 13349-13354. 

Masai E, A Ichimura, Y Sato, K Miyauchi, Y Katayama, M Fukuda (2003) Roles of enantioselective glutathione 5-transferases in cleavage of p-aryl ether. J Bacteriol 185 1768-1775. 

Akiyama, T. Magara, K. Matsumoto, Y. Meshitsuka, G. Ishizu, A. Lundquist, K. Proof of the presence of racemic forms of arylglycerol-p-aryl ether structure in lignin studies on the stereo structure of lignin by ozonation. J. Wood Sci. 2000, 46, 414-4-15. 

Figure 1. Structure of phenylcoumaran (I) and arylglycerol P-aryl ether II)...

Biphenyl structures and a-carbonyl-p-aryl ether structures, which are both assumed to be present in native lignin with a higher abundance than coniferyl alcohol structures (22,23) and both considered to be important leucochromophores, were not observed among die products, presumably because they are not present in spruce lignin as end groups. Both these types of structures are very stable and unlikely to be structurally changed during mild acid hydrolysis (24). 

Table 9 shows the relative hydrolysis rate of lignin models reported by Johansson and Miksche [302]. Both a- and P-aryl ether hydrolysis were enhanced by the presence of a phenolic hydroxyl groups. It is evident that a-aryl ether is much more reactive than the P-ether by a factor of 25 and 65... 

P-Aryl ether. Solvents were shown to significantly affect the hydrolysis rate of p-aryl ether dimers (29), being higher in aqueous dioxane than in water or aqueous ethanol media [306]. Both dioxane and ethanol favor the C6C2 enol ether (55) formation, especially at high temperatures [307]. 

The alkaline cleavage of a- and P-aryl ether linkages as well as condensation reactions have been extensively studied using lignin model compounds [22,24,27,32,277,326-343], but it is still not entirely clarified in the degradation of wood lignin. 

On the other hand, alkaline cleavage of the etherified P-aryl ether unit (47) proceeds by an intramolecular displacement mechanism (D). Under kraft cooking conditions, the p-aryl ether cleavage of phenolic type could be 12-50 times faster than that of the etherified type depending on the hydroxide and sulfide ion concentrations [331]. 

Etherified units. Consistent with the generally accepted mechanism (D, Fig. 14), the erythro dimer of etherified P-aryl ether was about four times more reactive than the threo isomer [350]. The ether cleavage, besides being enhanced by increasing alkalinity, was facilitated in the presence of monoetha-nolamine [351] or in a DMSO-potassium-tertiary butoxide solution [330]. 

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